Field of the Invention
This invention relates generally to a system and method for portable acoustic thermography of in situ turbine blades and, more particularly, to a system and method for portable acoustic thermography using a portable system that is enclosed, compact, and utilizes a spring loaded clamping mechanism to ensure that the turbine blade being inspected is adequately clamped for inspection.
Discussion of the Related Art
The world's energy needs continue to rise which provides a demand for reliable, affordable, efficient and environmentally-compatible power generation. A turbine engine is one known machine that provides efficient power, and often has application for an electric generator in a power plant, or engines in an aircraft or a ship. A typical gas turbine engine includes a compressor section, a combustion section and a turbine section. The compressor section provides a compressed air flow to the combustion section where the air is mixed with a fuel, such as natural gas, and ignited to create a hot working gas. The working gas expands through the turbine section and is directed across rows of blades therein by associated vanes. As the working gas passes through the turbine section, it causes the blades to rotate, which in turn causes a shaft to rotate, thereby providing mechanical work.
Maintaining the structural integrity of the blades in a turbine is important for proper operation of the turbine. Thus, it is very important to periodically check the blades for signs of deterioration, such as cracks and defects. One known technique for testing for material defects in the blades includes treating the blades with a dye penetrant so that the dye enters any crack or defect that may be present. The blades are then cleaned, and the structure is treated with a powder that causes the dye remaining in the cracks to wick into the powder. An ultraviolet (UV) light source is used to inspect the material to observe locations on the component that fluoresces as a result of the dye. This technique is disadvantageous, however, because it is inspector intensive and dependent and requires the person to be skilled. Additionally, the dye does not typically penetrate tightly closed cracks or cracks that are not on the surface.
A second known technique for inspecting a component for defects employs an electromagnetic coil to induce eddy currents in the blade. The coil is moved around on the blade, and the eddy current pattern changes at a crack or other defect. When the eddy current pattern changes a complex impedance in the coil changes, which can be observed on an oscilloscope. This technique has the drawback that it is also very operator intensive, slow and tedious.
A third known technique employs thermal imaging of the component to identify the defects. Typically, a heat source, such as a flash lamp or a heat gun, is used to direct a planar pulse of heat to the surface of the component. The material of the component absorbs the heat, and emits reflections in the infrared wavelengths. Certain types of defects will cause the surface temperature to cool at a different rate over the defects than for the surrounding areas. A thermal or infrared imaging camera is used to image the component and detect the resulting surface temperature variation. Although this technique has been successful for detecting disbands and corrosion, it is ordinarily not successful at detecting vertical cracks in the material, i.e., those cracks that are perpendicular to the surface. This is because a fatigue crack looks like a knife edge to the planar heat pulse, and therefore no, or minimal, reflections occur from the crack, making the cracks difficult or impossible to see in the thermal image.
Thermal imaging for detecting defects in a material that is capable of detecting small cracks as well as tightly closed cracks is described in U.S. Pat. No. 6,399,948 issued to Thomas et al. on Jun. 4, 2002. However, this technique requires the material that is being inspected to be placed in a thermal imaging system. Thus, if the material to be inspected includes turbine blades, the blades must be removed from the turbine to be inspected. Removal of turbine blades is costly, time-consuming and labor intensive. A portable system and method for in situ inspection of turbine blades is described in application Ser. No. 14/480,733, entitled “Hardware and Method for Implementation of In Situ Acoustic Thermograph Inspections”, filed Sep. 9, 2014, assigned to the assignee of the present application and incorporated herein by reference. Known systems, such as the '733 system, present the problem of pinch points that endanger operators due to exposed moving parts. Systems with exposed moving parts are also difficult to handle because the exposed moving parts make it difficult to grasp. Furthermore, the clamping mechanism of this portable system does not reliably provide a flush interface between the system and the blade being clamped for inspection. Thus, there is a need in the art for a system that allows for in situ inspection of blades that is easy to handle and also provides a flush interface with blades being inspected.